Maybe time to buff Kassadin more?
He is still at the point of, MUST GET EARLY FED TO HAVE ANY PRESENCE. He could use some sort of buff that keeps him in a 1v1 against certain champs he should be able to fight against even if his early or mid game was lack luster.
I feel like his Q buff, while tiny did help his early harass a bit when comboed with E. However the buffs like that and the small R ratio (which really only makes its large difference of course at x4 ults) were more of a "here you go kassadin players, we see he isn't used much". The real reason he isn't used too much though is because... Let's face it... He has to perform amazing early game to perform well mid game, and has to perform amazing mid game to perform amazing late game.
While yes, your team can easily make up for your failures in lane as kassadin and even possibly get you fed themselves. As an assassin I think he is very off par with several assassins.
Kassadin's whole kit revolves around his ult in the end. Its the source of his mobility, and the most op (compared to other mobility abilities) of them all. However I think mobility only gets you so far depending on circumstances (yes i know not all champions are meant to excel everywhere...)
instead of a
against a team comp with atleast 1 hard cc in each of the champs